Japanese Laid-Open Patent Publication No. 2014-22341 (abbreviated as JP 2014-22341 A) discloses an ignition plug for ignition of air-fuel mixture by non-equilibrium plasma in a combustion chamber of an internal combustion engine. This ignition plug includes a center electrode, a bottomed cylindrical insulator enclosing a front end portion of the center electrode, and a metal shell holding therein the insulator, with a front end portion of the insulator protruding from a front end of the metal shell, so that non-equilibrium plasma is generated in a space around the insulator for ignition of air-fuel mixture. The ignition performance of the ignition plug is improved as the amount of generation of non-equilibrium plasma is increased with increase in the surface area of the front end portion of the insulator.
However, the larger the surface area of the front end portion of the insulator, the more susceptible the front end portion of the insulator is to heat of combustion gas. When the surface temperature of the heated insulator becomes excessively high, there arises a problem of pre-ignition due to heat from the insulator.
On the other hand, the larger the volume of the insulator becomes with increase in the surface area of the front end portion of the insulator, the more easily the heat can be radiated (dissipated) from the front end portion of the insulator through the insulator. When the surface temperature of the front end portion of the insulator becomes low, it is likely that carbon will be deposited on the front end portion of the insulator. Due to such carbon deposition, it becomes difficult to generate non-equilibrium plasma so that there arises a problem of deterioration in ignition performance.
The present invention has been made to solve the above-mentioned problems. An advantage of the present invention is an ignition plug capable of improving its ignition performance while preventing pre-ignition.